All available database fields are listed on the left-hand window border under "Database Fields". Drag on one of these fields in order to drop it into the mail text.
Later, when the mails are being compiled, these placeholders will be replaced by the database contents.
A special feature is the field [Salutation]: it inserts the personalized salutation (only with an Internal database).
This salutation is can be configured freely, see Salutations